The present invention relates to a film composed of polyphenylene sulfide. More particularly, the present invention relates to a polyphenylene sulfide film having excellent slitting property and winding workability in the film forming process and also having high surface flatness and slipping property.
Polyethylene terephthalate films have been commonly used as conventionally industrial films. In recent years, however, a demand has arisen for the films further improved in heat-resistance.
Aromatic polyamide films and polyimide films are known as typical examples of the films having high heat-resistance. These films, however, are very high in production cost, since the starting materials are expensive and also the solution casting method is the only available means for forming such films.
As the materials to which the melt film-forming method advantageous in terms of film-forming cost is appliable and which have relatively high heat-resistance, polycarbonate, polyester carbonate, polyacrylate, polysulfone, polyetherimide, polyethersulfone, etc. are known. These materials, however, are poor in mechanical strength when formed into a film because of their amorphous property.
Thus, the films relatively low in production cost and having excellent heat-resistance and mechanical strength, are strongly required at present, and poly-p-phenylene sulfide (PPS) films are attracting attention as a film having a high possibility of meeting such demand.
Regarding such PPS films, U.S. Pat. No. 4,286,018 discloses a heat set biaxially oriented poly-p-phenylene sulfide film comprising predominantly a plurality of recurring units of the formula ##STR1## produced by a process which comprises (a) casting amorphous poly-p-phenylene sulfide film, (b) stretching said amorphous film within the range of about 2.0 to 5.0 times at a temperature between about 80.degree. -120.degree. C., so that the film birefringence in the stretching direction is between 0.05 to 0.30, thereafter (c) stretching said film in a direction transverse to that of step (b) within a range of about 1.5 to 5.0 times at a temperature between about 80.degree. -150.degree. C., thereby producing a biaxially stretched film, and (d) heat-setting said biaxially oriented film at a temperature between about 180.degree. C. and the melting point.
Also, the operating conditions for forming such films are described in Japanese Patent Publication Nos. 59-5099 (1984), 59-5100 (1984), 59-5101 (1984), etc. The typical properties such as thermal, mechanical and electrical properties of such films are also known.
However, several problems remain to be overcome for realizing industrial production of such poly-p-phenylene sulfide films and practical use thereof as base films for various products.
For instance, in use of a poly-p-phenylene sulfide film as a base film for magnetic recording media or as a dielectric for capacitors, it is required that the film surface is flat and smooth for improving the electromagnetic transducing characteristics or dielectric properties, but an improvement of such surface flatness and slipperiness tends to cause a deterioration of slipping property of the film, thereby occurring troubles in handling of the film. Such antinomic phenomena can be improved to some extend by blending particles of a proper size in the film, but not enough study and disclosure have been made on this matter in connection to a poly-p-phenylene sulfide film.
According to the knowledges of the present inventors, the conventional poly-p-phenylene sulfide films tend to break when slit in the film-forming process, and also when the slitted film is wound up, there tends to take place non-uniform or irregular winding.
As a result of further studies for solving said problems, it has been found that by regulating the size and number of surface protuberances of a poly-p-phenylene sulfide film within the specified ranges, it is possible to obtain a flat and slippery film and also the winding workability in the film-forming process is greatly improved. The present invention was attained on the basis of such finding.